By definition, a Press Release is a kind of news item released by the company on whom the news is being reported. As such, you will have to compete with all the other press releases and hope that your will get picked up.
Why Will Your Press Release Be Trashed?
1) Unless you are writing a press release about Microsoft, Adobe, Sony or one of the other mega-companies, or at least about a company that is relatively well known, forget about it. Journalists and news editors receive thousands of press releases a day and there is no way that they will waste more than a quick glance. So why should you pay a company seven to eight hundred dollars for them sending your press release to thousands of editors and journalists when they are going to junk it anyway?
2) Unless you are announcing a truly revolutionary product or technology (which I assume you are not since you are only interested in using the professionally paid and written press release to boost your search engine rankings), whatever you make up or announce such as some new free deal or new portal offering something unique, your chances of being picked up are very slim. Yes, the PR companies will tell you that they have vast experience in writing Press Releases that will make yours stand out and get picked up but this is not accurate. The only way your PR is going to get picked up is if it truly is something out of the ordinary or something that no one has ever thought off.
3) Unless you plan to spend millions of dollars and then just by sheer volume your site will go ahead in the search engines, this is not a good option. Why? If you send a large number of press releases then these will be placed on the different PR companies websites main page. Since most of these home pages have a very high page rank, your site will get a boost. The question is whether this boost is worth the large amount of money you will put into the PR companies pocket. No it is not. Use that money to buy (though I do not recommend this) links from high-ranked websites and you will pay less and receive more benefit.
4) Any press release, even if it has been accepted, will remain on the different pages for just a short time, making the time and effort and even more so the amount of money you paid fruitless.
A press release is useful if you wish to contact the press and maybe get some free publicity, but it is of no use if you wish to use it as a tool in order to advance your website in the search engines.